This invention relates to the problem of finding a concealable, inexpensive and universally adaptable intrusion sensing device for doors and windows.
A recent development in door alarm systems is disclosed in U.S. Pat. No. 4,233,595 to F. Landkammer on Nov. 11, 1980. This development, is adaptable to almost all door lock systems and does not require movement of frame parts relative to each other. The apparatus disclosed in Landkammer has two pieces which are called supports joined together by a flexible cord with a pair of conductors therein. The first support is mounted on a door, and the second support is mounted on a door frame or in an area close thereto. The flexible cord is permanently fixed to the first support on one end and has an end piece which is inserted into the second support on the other end. The second support also contains two frangible conductive elements which connect to the conductors in the cord at one end and to a resistor at the other end. These strips are then situated under a large head of a screw. If one opens the door and puts tension on the cord, the conductors in the two strips will either be shorted together by the screw head, or one or both of the conductors in the strips will break. Either event will cause a change in resistance in a conductive loop resulting in the triggering of an alarm. Breaking the cord or shorting the conductors in the cord together will also trigger the alarm.
Landkammer, however, is not without its shortcomings. If an intruder opens the door slightly so as not to break any conductors, he will see the cord and can react accordingly. Also, with this alarm system, the user must remember to arm and disarm the system. Otherwise, an alarm will be triggered when there is a non-intrusive entry, or the alarm will not trigger when there is an intrusive entry. Arming the system requires opening the door and holding it slightly open, reaching inside the opening and inserting the end piece of the cord into the second support. Disarming also requires slightly opening the door, reaching inside and using a key to detach the cord's end piece from the second support. Another disadvantage of Landkammer, as can be seen above, is that the apparatus in this disclosure requires many parts such as two supports and an interconnecting cord.
A more recent development in the area of door alarm systems is disclosed in U.S. Pat. No. 4,390,867 to J. Queren on June 28, 1983. With this development, a flange projects from a backer plate into a latch bolt hole in a door frame. The flange is slightly spaced from the latch bolt when the latch bolt is in the latch bolt hole of the door frame. When the latch bolt is displaced toward the side of the hole, the flange is slightly bent, thereby closing a normally open circuit setting of an alarm. However, this alarm system is not easily installed, requires many parts and is not easily adaptable to locks that have been previously installed without this alarm system. This alarm system requires a backer plate and a reinforcement plate, which is in addition to the strike plate used in most door latch bolt locks. Conductors must also be connected to the reinforcement and backer plate.
Thus, the prior art discloses various door alarm systems which respond to intrusion; however, these door alarm systems are not without their shortcomings. Many of these systems require extensive modification of the door locks. Many other of these systems require numerous parts, are not concealable, and must be deactivated and reactivated for non-intrusive and intrusive openings, respectively.
There is, therefore, a need to provide an intrusion sensitive door alarm which is concealable, inexpensive, easy to install and adaptable to most door locks.
There is a further need to provide a passive sensor that is automatically armed when the door is locked and disarmed when the door is unlocked without requiring any additional action by the user.